The invention relates in general to a process and apparatus for separating a gas mixture for example, cryogenically in a separating plant, in which process feed gas is compressed to a first pressure and then introduced into a separation system in order to separate a first product stream.
Whereas the separating plant usually operates in the steady state, the invention is directed to conditions wherein the plant is beset with at least a partial loss of compressor function.
In such separating plants, various compressor systems can be used to compress the feed gas:
a single compressor, for example having a plurality of sequential stages
two or more serially-connected separate compressors
two or more compressors connected in parallel
comprehensive gas compression system which supplies not only the separation plant, but other large consumers (for example pressurized gas system, in particular compressed air network with compressor rail)
The preferred field of application of the invention is a cryogenic air separation process for supplying an integrated power station process (IGCCxe2x80x94Integrated Gasifier Combined Cycle Process). Energy production is served by a gas turbine system which has a gas turbine (gas turbine expander), a gas turbine compressor driven by the gas turbine and a combustion chamber. One or more products of the air separation are used in the energy production system. For example, oxygen produced in the air separator can be used to produce a fuel gas with which the combustion chamber is charged; in this case the oxygen serves in particular as oxidizing agent in coal or heavy oil gasification. Alternatively, or additionally, nitrogen introduced from the air separator into the gas turbine stream can be used by feeding it into the combustion chamber or the gas turbine or mixing it with the gas turbine exhaust gas between combustion chamber and gas turbine of the combustion chamber. In some cases, nitrogen can also be used to transport coal into a synthesis gas plant.
The gas turbine compressor supplies firstly the air required for the combustion process and secondly a portion of the feed air for the air separator (the xe2x80x9cfirst feed gas streamxe2x80x9d); it thus represents the xe2x80x9cfirst feed gas compressorxe2x80x9d in the context of the invention. In the process of the invention another portion of the feed air for the air separation (the xe2x80x9csecond feed gas streamxe2x80x9d) is compressed by a separate air compressor (the xe2x80x9csecond feed gas compressorxe2x80x9d), which is driven independently of the gas turbine system, for example by means of an electric motor or a steam turbine. The ratio between the first and second feed air stream can in principle have any value. In practice, it is generally 30:70 to 70:30, preferably 40:60 to 60:40.
If the gas turbine fails owing to an operating fault, after a very short time the appropriate feed air rate for the air separator is lacking. Together with the fall in air rate and air pressure, the load of the rectification column(s) of the air separator also falls, the liquid (holdup) drops from the plates or the packings to the bottom and all product purity grades are lost. Hitherto, no process has been known for maintaining the separation process in the air separator after such a fault. Further operation of consumers of the air separator, for example the gasification plant, is only possible temporarily using an external emergency supply involving expensive pressurized tanks and evaporating liquid products.
It is an object of one aspect of the invention, therefore, to provide a process and apparatus for gas separation of preferably the type mentioned at the outset which can operate further in the event of loss of one of the two feed gas compressors. Upon further study of the specification and appended claims, further objects and advantages of this invention will become apparent to those skilled in the art.
According to one aspect of the invention, in the event of loss or partial loss of the compressor system, a first auxiliary stream which has approximately the composition of a first product stream or approximately the composition of the feed gas is compressed to approximately the first pressure and is recirculated to the separation plant.
In the context of the present invention, the feed gas rate which is lacking owing to a fault in the compressor system is at least partially replaced by an auxiliary stream from a different source. In this manner, the decrease in the rate of feed gas is at least partially compensated and the separation process (for example a rectification) can be maintained. It is then possible to bring the separation plant in a controlled manner into a partial load mode and thus to decrease the auxiliary stream rate, if appropriate to zero.
The designation xe2x80x9capproximatelyxe2x80x9d with respect to the composition of the first auxiliary gas denotes a deviation of the proportion of each component of a maximum of 5 mol %, preferably a maximum of 1 mol %. In the specification of pressure, xe2x80x9capproximatelyxe2x80x9d permits deviations of the order of magnitude of the pipeline losses and of the flow losses.
The other source from which the auxiliary stream comes can be, for example, the separation plant itself. In this case, in the event of a fault, at least one first product stream of the separation plant is recirculated to the separation plant. Especially when the product or products are produced in any case under pressure, the product stream to be recycled can be taken off downstream of the existing product compressor or from an intermediate stage of the product compressor, if appropriate expanded to the feed gas pressure (the xe2x80x9cfirst pressurexe2x80x9d) and fed back to the separation plant, for example to the rectification column or to a rectification column of an air separator.
Thus, although the product rate is also decreased (in the event of total loss of the compressor system the separation plant can under some circumstances initially no longer deliver any product at all), this disadvantage is, however, more than compensated for by the virtually uninterrupted further operation of the separation plant. If there is such a need, the consumers can be supplied by an emergency supply system (backup system) in the period of decreased or absent product delivery. Such a system has, for example, a storage unit in the form of at least one liquid tank and/or a gas pressurized store. The storage unit can be filled from outside the separation plant (for example using tanker vehicles or pipelines) and/or by product produced in usual operating mode of the separation plant. If the storage unit is in any case at a pressure which is at least approximately equal to the xe2x80x9cfirst pressurexe2x80x9d (the outlet pressure of the compressor system), there is no requirement for a separate pressure boosting system for recycling the corresponding auxiliary stream to the separation plant.
If such an emergency supply system is present, an emergency supply stream produced there can also be recirculated to the separation plant, instead of to the consumer, alternatively or additionally to one or more product streams from the separation plant.
Preferably, in the event of loss or partial loss of the compressor system, two product streams of different composition are recirculated to the separation plant, for example a nitrogen stream and an oxygen stream in the case of air separation. As a result, it is possible within the scope of modern process control technology to adapt the composition in the recycle stream substantially to the feed gas. In the optimum case, not only the total rate of the recycled product streams, but also their relative rates, are set so that they correspond as far as possible exactly to the missing feed gas rate. Thus the loss of the first feed gas compressor has no direct effect on the separation processxe2x80x94the separation plant experiences virtually nothing from this serious operating fault in the compression of the feed gas. Obviously, the two recycled streams (xe2x80x9cauxiliary streamsxe2x80x9d) can also be formed in part or completely by emergency supply streams.
When a first and a second feed gas compressor are connected in parallel in the compressor system, the inventive emergency connection is used, especially when one of the two feed gas compressors is lost in whole or in part. In this case the two feed gas compressors can take in two part-streams of the feed gas, either via a shared line, in which case this shared line has, for example, one filter; or the inlet of each of the two feed gas compressors can be connected to a separate intake line which is independent of the intake line of the other feed gas compressor.
In principle it is possible to introduce the recycled auxiliary stream or streams separately into the separating device, for example into a rectification column. However, it is more expedient if the recirculated product stream, or the recirculated product streams, are mixed with the compressed second part-stream of the feed gas and the mixture is introduced jointly into the separating plant, for example into the coldbox of an air separator. The mixing can be performed upstream or downstream of a purifying device for the feed gas. For example, the recycled product stream or streams can be introduced directly into a cooling device for the feed gas, for example a direct contact cooler.
To compress the recirculated product streams, preferably those devices are used which, in fault-free operation, serve to produce the required product pressure. These can be, for example, a multistage gas compressor (external compression) and/or a device for internal compression that comprises a liquid pump and means for vaporizing the product stream brought to pressure in the liquid state. In the case of external compression, the product stream to be recycled is taken off downstream of the gas compressor or from the outlet of one of its intermediate stages. In the case of a pure internal compression, takeoff is performed downstream of the heat exchanger for product vaporization, for example at the warm end of a main heat exchanger obviously, internal and external compression can also be combined in the same product stream. In the case of air separation, nitrogen is frequently externally compressed and oxygen is subjected to internal compression.
An important field of application of the invention is, as already stated, air separation plants, in particular cryogenic air separation plants. In these the feed gas is formed by air, the separation plant by an air separation plant and the recirculated product stream or the recirculated product streams by a nitrogen product stream and/or by an oxygen product stream and/or by corresponding streams from an emergency supply system. In the case of a cryogenic air separator, the xe2x80x9cseparation plantxe2x80x9d comprises the entire coldbox including the rectification column(s) for nitrogen/oxygen separation and the main heat exchanger. It is preferably a classic Linde double-column plant. However, the invention can be used in other two-column processes, in single-column processes or in processes having three or more columns. Particularly advantageously, the invention can be used in air separation processes in connection with IGCC plants, as described, for example, in DE 2434238, DE 2503193, Springmann, Linde-Berichte aus Technik und Wissenschaft, 51/1982 pp. 55-65, Rottman, Schonpflug, Sauerstoffversorgung fxc3xcr Kohlevergasungs-Kraftwereke (Combined Cycle Process) [Oxygen supply for coal gasification power stations (Combined Cycle Process)], BMFT-Forschungsvericht T 82-018, Beysel, Sommerfeld, VGB Conference xe2x80x9cKohlevergasung 1991xe2x80x9d [Coal gasification 1991], 16-17.05.1991, Dortmund, Lecture D1, EP 758733 B1, DE 19818308 A1, German Patent Application 10052180 which does not have an earlier priority than the present application, and the applications corresponding thereto, and German Patent Applications 10103968 and 10103957 which do not have earlier priority than the present application, and the applications corresponding thereto.
In principle, the inventive product recycling can be used in any gas separation process having at least two feed gas compressors operating in parallel, in particular in any air separation process having at least two parallel air compressors. Furthermore, the product recycling can also be used in separation processes having a single compressor. If this fails completely, however, the entire production must first be recirculated, that is to say no end products are available for the consumer or consumers. However, under some circumstances, this can nevertheless be expedient in order to be able to shut down the separation plant in a controlled manner or to keep it in operation (so to speak in idling mode).
The invention also relates to an apparatus for gas separation, in particular for the cryogenic separation of air according to Patent Claim 13.